This invention relates to a map display apparatus for use in a navigation system for measuring the position of a mobile body and reporting the present position to a user, and more specifically to a bird's-eye view map display apparatus which provides a map in a more comprehensible way to the user.
A navigation apparatus mounted to a mobile body processes information from a variety of sensors to measure the position of the mobile body and reports the position to a user. This navigation apparatus comprises position measurement means for measuring the absolute position of a mobile body, memory means for storing map data constituted by two-dimensional vector data obtained by projecting points on the ground such as roads and buildings on a plane divided into meshes by universal transverse mercator projection and character data accompanying the two-dimensional vector data, input means for receiving commands from the user, and display means for reading the necessary vector data from the map mesh stored in the memory means in accordance with the command inputted from the input means and conversion processing the data to display the map on a display. Here, data conversion processing includes movement conversion for changing the display position of the map, reduced scale conversion, such as enlargement and reduction, used for displaying the map in an arbitrary reduced scale and rotation conversion for changing the displaying direction of the map. By means of these processings, a plan view map depicting the ground surface directly overhead by normal projection is displayed on the display.
In navigation apparatuses, according to the prior art, plan view map display which depicts a map by normal projection directly overhead has been employed to display the map. When two points spaced apart from each other are simultaneously displayed, therefore, a reduced scale becomes unavoidably great and detailed information cannot be displayed. One of the means for solving this problem is a bird's-eye view display system which displays a map when points having a certain height from the ground surface are looked down obliquely from afar on a plane. In order to apply this bird's-eye view display to the navigation apparatuses, the following problems must be solved.
First, in the case of the bird's-eye view display which displays a broader range of regions than the plan view map, a reduced scale becomes great at points far from the start point, so that a greater quantity of information is displayed. According to the prior art systems, character strings of those regions in which the reduced scale becomes great are not displayed or the character strings in the proximity of a view point are merely displayed at the upper part. For this reason, fall-off of characters and overlap of character strings are unavoidable, and recognizability of the characters by the user drops.
Secondly, background data and character data are constituted in the map data base so that display quality attains the highest level when the plan view map is displayed. Therefore, in the bird's-eye view map display displaying a broader range of regions, the frequency of the occurrence that the same character strings are displayed at a plurality of positions becomes higher. Since no counter-measure has been taken in the past for the same character string, the same character string is unnecessarily displayed and this unnecessary character string hides the roads and other background data. In consequence, display quality gets deteriorated.
Thirdly, though a route to the destination is displayed in superposition with the map in a different color from those of the background roads, all the route data are displayed with the same line width in the past because the concept of the road width does not exist in the vector data expressing the routes. However, because the map is expressed three-dimensionally in the bird's-eye view display, the feel of three-dimensional depth will be lost if all the routes are displayed by the same line width.
In the fourth place, in the display of a driving orbit, it has been customary in the prior art to store the position information of driving in a certain distance interval and to display the points representing the driving orbit on the basis of the position information so stored. When the driving orbit is displayed by the method of the prior art system on the bird's-eye view map, however, the gap of the point strings representing the orbit is enlarged in the proximity of the view point at which the reduced scale becomes small, and the user cannot easily recognize which route he has taken. The gap of the dot strings becomes unnecessary narrow, on the contrary, at portions away from the view point at which the reduced scale becomes great, and the roads and the character strings as the background information are hidden. Therefore, the user cannot easily recognize the map information, either.
In the fifth place, pattern information, e.g. solid lines and dash lines, used for displaying the vector information such as roads, railways, administrative districts, etc., and pattern information, e.g. check and checkered patterns, used for displaying polygons representing water systems, green zones, etc., are registered to the map data base. When the map containing these pattern information is displayed by bird's-eye view, the prior art systems execute not only perspective conversion of each apex coordinates constituting the lines and the polygons but also perspective conversion of the patterns for displaying the map. Therefore, the processing time becomes enormously long, and the time required for bird's-eye map display gets elongated.
In the sixth place, in order to prevent dispersion of the map data displayed near an infinite remote point called a "vanishing point" in the bird's-eye map display, the display region is limited to the foreground region by a predetermined distance from the vanishing point and artificial background information such as virtual horizontal line and sky are displayed at the depth in the prior art system. However, these artificial background information in the prior art systems have fixed patterns or fixed colors and do not match the surrounding conditions.
In the seventh place, when the bird's-eye view map display and the plan view map display are switched, the user cannot easily discriminate which of them is actually displayed when the number of objects plotted is small. Moreover, the user can operate and change the position of the view point in the bird's-eye view map display, and the map region actually displayed greatly changes depending on the position of the view point and on the direction of the visual field. In the prior art systems, however, there is no means for providing the information of the position of the view point, etc., even when the position of the view point and the direction of the visual field are changed, and the systems are not easy to handle.
In the eighth place, when the bird's-eye view map is displayed, the map displaying direction is set in such a manner that the image display direction coincides with the driving direction, as described, for example, in JP-A-2-244188. When the destination is set, the driving direction and the direction of the destination are not always coincident, so that the destination disappears from the screen. Accordingly, there remains the problem that the user cannot recognize the map while always confirming the direction of the destination.
In the conventional bird's-eye view map display, in the ninth place, even when a map information density is low in a certain specific direction or when a specific direction comprises only information having specific attributes, the display position of the view point, that is, the display position of the present position, does not change on the screen. In other words, there occurs the case where a large quantity of information, which are not much significant, are displayed on the display region having a limited area, and the information cannot be provided efficiently.